Coordinated Control of Power and Current of MMC-HVDC and Circulation Suppression in Unbalanced Power Grid

During the operation of the modular multilevel converter (MMC) for the high-voltage direct current (HVDC) transmission system, the negative sequence component will be generated when an unbalanced fault occurs in the grid, which will cause fluctuations in active and reactive power and the circulating current will flow through the bridge arm. Based on complex resonance controllers (CRC), this paper firstly proposes a fast and accurate separation technology for the positive and negative sequence voltages. Secondly, in order to control three objectives of negative sequence current, active power and reactive power fluctuations flexibly and harmoniously, a general positive and negative sequence current calculation instruction by adjusting its trade-off coefficient in the α-β frame is used. Thirdly, a circulating current suppressor based on resonant vector controllers (RVC) is presented to reduce the loss of MMC. Finally, the effectiveness of the proposed strategy is verified by the simulation results.